The present invention relates to a magnetoresistive element input circuit, and more particularly, to an input circuit or preamplifier of an integrated circuit for driving a magnetoresistive head.
A magnetic head formed by a magnetoresistive (MR) element is used in magnetic recording devices, such as magnetic tape devices and hard disk devices, to increase the recording density. An input circuit supplies the magnetoresistive element with a bias current to read data signals from a recording medium and amplify the signals.
FIG. 1 is a schematic circuit diagram showing a first example of a prior art MR element input circuit 10.
The input circuit 10 includes a first resistor 12 connected between an MR element 11 and a high potential power supply (first power supply) V1, a second resistor 13 and a current source 14 connected in series between the MR element 11 and a low potential power supply (second power supply) V2, and a differential amplifier 15 having two input terminals connected to the two terminals of the MR element 11. The first and second resistors 12, 13 have the same resistance. A capacitor 16 is connected to a node N1 between the second resistor 13 and the current source 14. The capacitor 16 substantially equalizes the alternating current impedance at the two terminals of the MR element 11.
In the first example, since the input impedances at the two terminals of the MR element 11 are the same, the two signals respectively provided to the differential amplifier 15 include external noise of the same phase. Accordingly, the S/N ratio of the signal output by the differential amplifier 15 is improved and the affect of the external noises is reduced.
However, in the first prior art example, the current source 14 constantly supplies the MR element 11 with a bias current. Thus, the power is consumed even when the input circuit 10 is inactive.
Accordingly, an MR element input circuit 20 (second prior art example) shown in FIG. 2 has been proposed to reduce power consumption. Like or same reference numerals are given to those components that are the same as the corresponding components of the first prior art example.
The second example further includes a third resistor 21, a switch element 22, and an NPN transistor 23. The transistor 23 is connected between a first resistor 12a and a high potential power supply V1. The base of the transistor 23 is connected to the high potential power supply V1 via the third resistor 21 and to the low potential power supply V2 via the switch element 22. The sum of the resistance of the first resistor 12a and the ON resistance of the transistor 23 is the same as the resistance of the second resistor 13.
In the input circuit 20, the current Im flowing through the MR element 11 is inhibited when a control signal (not shown) closes the switch element 22 and deactivates the current source 14. This reduces power consumption when the input circuit 20 is inactive.
However, in the second prior art example, when the input circuit 20 switches from an active state to an inactive state (i.e., when the switch element 22 goes on and the current source 14 is deactivated), the charge stored in the capacitor 16 may cause an excessively large amount of current to flow through the MR element 11.
Further, when the input circuit 20 is active, the charge stored in the capacitor 16 is spontaneously discharged, for example, through the second resistor 13, the MR element 11, and the differential amplifier 15. Therefore, when the input circuit 20 shifts from an inactive state to an active state (i.e., when the switch element 22 goes off thereby activating the current source 14 and recharging the capacitor 16), a current exceeding a tolerable level may flow through the MR element 11, as shown in FIG. 3.
Additionally, in the second prior art example, a stable signal cannot be obtained from the input circuit 20 unless the capacitor 16 is charged. The recharging time of the capacitor 16 (in FIG. 3, the time required to stabilize voltage VN1 (the voltage at node N1)) thus determines the time for the input circuit 20 to switch from an active state to an inactive state, or the transition period. Since the required capacitance of the capacitor 16 is relatively large (e.g., 1 xcexcF) to stabilize operation, the transition period is long.
To solve these problems, an MR element input circuit 30 (third prior art example) shown in FIG. 4 has been proposed. Switch elements 31, 32 are respectively connected to the two terminals of the MR element 11 to disconnect the flow of current Im through the MR element 11. However, the number of elements connected between the MR element 11 and the high potential power supply V1 and between the MR element 11 and the capacitor 16 is increased in the third embodiment. This makes it difficult to match the alternating current impedance at the two terminals of the MR element 11. The differential amplifier 15 may thus be affected by external noises. Further, since the impedance at both terminals of the MR element is high when the input circuit 20 is inactive, an undesirable current may flow through the MR element 11.
It is an object of the present invention to provide an input circuit that is not overly affected by external noises, consumes less power when in an inactive state, and has a short transition period for shifting from an inactive state to an active state.
To achieve the above object, the present invention provides a magnetoresistive element input circuit having an active mode and an inactive mode. The input circuit includes a first resistor connected between a magnetoresistive element and a first power source. A first current source is connected between the magnetoresistive element and a second power source to supply a DC bias current to the magnetoresistive element in the active mode. A second resistor is connected between the magnetoresistive element and the first current source. A capacitor is connected to a node between the second resistor and the first current source and to the first power supply. A differential amplifier is connected to the magnetoresistive element. A voltage supply circuit is connected to the node to supply the node, when the input circuit is in the inactive mode, with a voltage substantially equal to that supplied to the node when the input circuit is in the active mode.